1. Field of the Invention
This invention relates to a method for controlling the grain size distribution in cast parts made from nickel-based superalloys. Such methods of this type, generally, employ the use of different inoculant concentration levels to balance the differences in cooling rates that occur at different regions of the cast part in order to achieve the desired microstructure of the cast part.
2. Description of the Related Art
It is known, in investment casting (commonly known as, lost wax casting) to employ homogeneous or heterogeneous nucleation techniques. In particular, with respect to the homogeneous nucleation technique, the alloy begins to nucleate even when there is no external solid phase present. However, this homogeneous technique is very difficult to achieve due to the surface tension of the liquid metal. The surface tension of the liquid metal typically does not allow for the nucleation to occur. Consequently, the heterogeneous nucleation technique is the preferred technique.
In this heterogeneous technique, the nickel-based superalloys generally require the usage of an external solid phase, known as grain refiners, to help increase the number of nucleation sites, thereby resulting in a more finely-grained microstructure. In the terminology of the casting industry, these grain refiners are called "inoculants". The inoculation process is implemented by mixing the inoculant into the ceramic slurry used to form the first layer of the ceramic mold. A second technique for implementing the inoculation process is to add a coating on the inner surface of the ceramic mold, where the coating layer consists of a mixture of inoculant and binder. Typically, the binder is any suitable binder.
The grain size of the microstructure varies significantly from thin sections to thick ones in production cast parts. FIG. 1 shows a section of an investment cast engine component, which exhibits a large variation in thickness and grain size throughout the part. Generally such an investment cast engine component is cast using the heterogeneous nucleation technique. This technique consists of subjecting the component to an equal concentration of inoculant on all of the mold surfaces. However, while an equal concentration of inoculant is ideal for a constant thickness part, the complex design of the engine component adversely affects the grain size distribution of the alloy. In particular, the thinner sections tend to have finer grain size while the thicker sections usually include larger grain sizes. These variations in grain size adversely affect the mechanical properties of the component. Therefore, a more advantageous method, then, would be presented if such variations in grain size could be controlled.
It is apparent from the above that there exists a need in the art for a method which is capable of creating nucleation sites, and which at least equals the nucleation site creation characteristics of known nucleation techniques, particularly those of the highly advantageous heterogeneous nucleation technique, but which at the same time is capable of controlling the grain size. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.